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>> The US military is testing to see if Islamic State tried to attack using chemical weapons. The militants fired a rocket that may have been filled with a chemical agent near Qayyarah West base. Hundreds of US forces are working there to prepare an airfield ahead of Iraq's offensive to retake the city of Mosul from IS.

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Reuters' correspondent in Baghdad, Stephen Kalin.>> One US official told Reuters that a black tar-like substance was taken from the side of the attack and tested. First came back as positive for mustard gas, and the second time it came back negative. So they're still investigating whether this was a chemical attack.

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>> The US military has used air strikes to repeatedly hit the group's chemical weapon stores. Experts have warned the militants might use the material in the fight for Iraq's second city.>> There have been a number of chemical attacks documented by Islamic State, both in Iraq and next door in Syria.

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A lot of those have been chlorine gas, which is easier to acquire. But there have been some reports of mustard gas, especially the Kurdish Peshmerga forces up in the north have said they've experienced this.>> The attack comes as Iraqi forces successfully recapture a key town nearby. With Islamic State kicked out, residents in celebrated the arrival of the military.

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e gain brings them one step closer to the main push on Mosul later this year.